The present invention relates to vehicle navigation systems and more particularly, the present invention is an improvement that relates to the use of GPS in a navigation system to determine the position of a vehicle relative to a road network represented by data contained in a geographic database used by the navigation system.
In-vehicle navigation systems provide a variety of useful features to end users (i.e., the drivers and/or passengers of the vehicles in which the navigation systems are installed). Included among the features that are provided by some in-vehicle navigation systems are route calculation, route guidance, emergency roadside services, electronic yellow pages, and so on.
Some of the features provided by in-vehicle navigation systems require that the position of the vehicle be determined. There are several considerations related to determining the position of the vehicle. For example, a GPS system may be used to obtain the geographic coordinates of the vehicle. However, the geographic coordinates only indicate the position of the vehicle relative to the surface of the earth. For some of the features provided by in-vehicle navigation systems, a means is required to determine the vehicle position relative to the road network which is represented by the geographic data used by the in-vehicle navigation system. In other words, if the vehicle is located on a road segment, a means is required to identify the road segment, and optionally the position and direction along the road segment. Once the vehicle position is determined relative to the road segments in the geographic area, programming in the in-vehicle navigation system can be used to provide various features, such as determining a route to a desired destination, providing maneuvering instructions for reaching the destination, identifying the closest restaurant or gas station relative to the vehicle position, displaying a map of the area around the vehicle, and so on.
Some of the features provided by in-vehicle navigation systems require that the vehicle position be updated continuously in real time as the vehicle is driven along roads in a geographic region. For example, in some navigation systems, a map display feature may continuously update a graphic display of a map of the area through which the vehicle is traveling on a display of the navigation system. Also, a route guidance function provided by some navigation systems may provide maneuvering instructions to the driver as the vehicle approaches the location at which the maneuver is required. These kinds of functions require that the data identifying the vehicle position be updated continuously or regularly as the vehicle moves in a geographic region.
There are prior methods for determining a vehicle position relative to a road network represented by data in a geographic database. However, a need for improved methods continues to exist.
Prior to May 1, 2000, the accuracy of GPS systems used in vehicle navigation systems was limited due to an intentionally included error, referred to as xe2x80x9cSelective Availability.xe2x80x9d Although GPS systems were subject to other kinds of errors (such as atmospheric errors), the error due to Selective Availability was substantially greater than the other kinds of errors. Accordingly, the other kinds of errors were difficult to detect in the presence of the Selective Availability error. Furthermore, there was little or no benefit to addressing these other errors because the error due to Selective Availability limited the accuracy of GPS systems anyway.
Since the Selective Availability error has been removed, GPS systems are able to determine a position of an object, such as a vehicle, with greater accuracy. However, there continues to be a need to determine a vehicle position with still greater precision and accuracy.
To address these and other objectives, the present invention comprises a method implemented by a navigation system in a vehicle for determining the position of the vehicle relative to a road network. The navigation system uses a geographic database that contains data that represent positions of roads upon which the vehicle travels. Using the output from a GPS system, and optionally outputs from other sensors, the navigation system matches the positions of the vehicle to the locations of the roads represented by the data contained in the geographic database. Upon detecting an event from which the position of the vehicle with respect to the roads represented by the data contained in the geographic database can be determined with a relatively high degree of accuracy, a correction factor is determined. The correction factor is an offset (i.e., a distance and direction) of the GPS position reported during the event to the known-to-be-highly-accurate position. The correction factor is then used to adjust subsequently obtained GPS readings for a limited period of time, i.e., while the same atmospheric conditions apply. This period of time may be 10-20 minutes. Upon detection of another event from which the position of the vehicle with respect to the roads represented by the data contained in the geographic database can be determined with a relatively high degree of accuracy, a new correction factor is determined and used.
FIG. 1 is a block diagram of a navigation system installed in a vehicle.
FIG. 2 is a flowchart that illustrates operation of the vehicle positioning application of FIG. 1.
FIG. 3 is a flowchart that illustrates operation of the correction factor program of FIG. 1.